wallet: rename ProcessDescriptorImport to ImportDescriptor and add ProcessDescriptorsImport

Rename ProcessDescriptorImport() to ImportDescriptor().
ImportDescriptor() takes a single descriptor request and imports it into the wallet.

Add ProcessDescriptorsImport() (descriptors in plural) to handle wallet locking and rescanning
over a vector of ImportDescriptorRequest items. It is also in charge of collecting all responses
of imports from ImportDescriptor.

This allows a next commit to extract ImportDescriptor() and
ProcessDescriptorsImport() out of the RPC code so they can be used by
other future interfaces.